Process for the agglomeration of water-swellable polymers by means of sinter granulation

ABSTRACT

The present invention relates to granular materials of water-swellable polymers having a content of water-swellable polymer and agglomeration auxiliary agent as main component, and optionally further additional non-agglomerating components, the invention further relates to the method for the manufacture of said material, to its use as absorbents, and to disposable articles comprising these granular materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to granular material of water-swellablepolymers, a method for the manufacture thereof and its use for theabsorption of aqueous solutions, especially body liquids, such as bloodand/or urine.

2. Description of Related Art

Today, water-swellable polymers are being used in various manners,particularly in the hygienic field for the absorption of body liquids,such as blood and/or urine. The water-swellable polymers must be capableto bind large amounts of liquid (water or body liquids) within a veryshort period of time. These known water-swellable finely dividedpolymers, however, tend to "block", i.e. when contacting water oraqueous solutions the outer layer of the bed becomes sticky and thusprevents the liquid from further penetrating into the innermost of theabsorbent.

The water-swellable powder-like finely divided polymers are obtained ina more or less wide spectrum of grain sizes, depending on the method ofmanufacture, said spectrum starting from fine sizes to coarser grains upto coarse-grains. A typical grain spectrum for a water-swellable polymerground after drying is in the range of from 10 to 800 μm, whereby thesize fraction of 150 to 630 μm is used as absorbent for practicalpurposes. The fine powder obtained having a grain size of about 10 to150 μm is too fine for the use as absorbent and is therefore to beregarded as waste, and, moreover, is an annoyance.

Therefore it is the object of the present invention to provide suchwater-swellable, powderlike polymers in a novel appearance not havingthe disadvantage of blocking, having the capacity of rapidly absorbinglarge amounts of liquids and which additionally may be produced in aneasy and economical way.

SUMMARY OF THE INVENTION

According to the present invention this object is achieved by a granularmaterial of a water-swellable polymer obtainable by agglomeration withagglomeration auxiliary substances, which, due to their ability to meltand soften, are capable of agglomerating the finely divided polymerparticles.

The increase of grain size by agglomeration of finely dispersedmaterials is known per se. In order to build up granular material frompowder particles many processes are available (Ullmann Encyclopedia ofTechnical Chemistry, 4th edition, vol. 18, page 157; Chem. Ind. 53(1981), pages 37-41), said processes include sintering agglomeration andmelting agglomeration. However, pelletizing of powder-like,water-swellable polymers has not been used in the art, up to now.

Surprisingly, the powdery water-swellable polymers may be agglomeratedby heating mixtures having powder-like, meltable, thermoplastic andthermosetting substances as agglomeration auxiliary agents, whereby thestability of the agglomerates is increased and thus processing and useis considerably improved. Furthermore it is surprising that the granularmaterial manufactured in this way does not tend to "block" but mayabsorb large amounts of liquid within a short period of time.

DESCRIPTION OF PREFERRED EMBODIMENTS

This surprising improvement of their properties can be recognized in allagglomerated water-swellable polymers, e.g., cross-linked homopolymers,copolymers and terpolymers of the acrylic and methacrylic acid, graftpolymers of acryl derivatives on starch and cellulose products,polyurethanes, saponification products of cross-linked homopolymers andcopolymers of the (meth)-acrylonitrile, the (meth)-acrylamide andacrylic esters, and copolymers of isobutylene and maleic acid anhydride.The polymers may contain further ionomeric and non-ionic momomers, suchas, e.g., acrylamidopropanesulfonic acid, vinyl phosphonic acid, vinylsulfonic acid, dialkylaminoalkyl(meth)-acrylates,dialkylaminoalkyl(meth)-acrylamides, quaternization products of monomershaving tertiary amino groups, monomers containing one or more allylgroups, (meth)-acrylamide, alkyl(meth)-acrylamides,methylenebisacrylamide, monoacrylic-acid alkyl ester, diacrylic-acidalkyl ester, and triacrylic-acid alkyl ester, hydroxialkyl(meth)-acrylicester, and compounds having epoxide groups. The water-swellable polymersmay be present, totally or partially, as salts of ammonium, alkali orearth alkali alone or in admixture.

The manufacture of these polymers is known. They may be manufacturedaccording to different methods: such as, e.g., solvent polymerization,precipitation polymerization, suspension polymerization, beadpolymerization or graft polymerization.

Any powder-like, synthetic or natural meltable thermoplastic orthermosetting substances alone or in admixture may be used asagglomeration auxiliary agents, said substances having amelting/softening/point/range. The following agents are meant to beexamples which shall not restrict this application:

Suitable synthetic agglomeration auxiliary agents are: polyolefins andtheir copolymers and terpolymers, polyamides and copolyamides, polyesterand copolyester, poly(meth)-acrylates and poly(meth)-nitriles and thecopolymers and terpolymers thereof, polyvinyl esters and allylesters andethers, respectively, the copolymers and saponification products as wellas actetals thereof, polyalkylene glycols, polyalkylene oxides, andpolyalkoxylated compounds, PVC and the copolymers and terpolymersthereof (e.g., with vinyl acetate or maleic acid esters) and itsmixtures with other polymers, polystyrene, styrene-acrylonitrile andacrylonitrile-butadiene-styrene polymers including their mixtures.Polycarbonates and polyurethanes in the form of powder are suitable,too. Furthermore, epoxide resins, aminoplasts, phenoplasts alone or incombination with a hardener may be used, too.

Natural substances, their derivatives and conversion products areexcellent agglomeration powders, too, such as, cellulose ester, shellac,colophony and colophony ester and hydrogenated and disproportionatedcolophony, and natural thermoplastic material on the basis ofhydroxybutyric acid and hydroxyvaleric acid. Further substances are thegroup of fatty acids and fatty acid derivatives, e.g., the esters,amides and amines of fatty acids and metallic soaps.

It is possible to add as additional non-agglomerating componentsseparating agents such as pyrogenic SiO₂ (Aerosil) or Al₂ O₃ or calciumsilicate ("Silcasil") in amounts of 0.1-1%-wt of the powder mixture tobe agglomerated.

The agglomeration is carried out after continuous or discontinuousadmixture of the powder-like, water-swellable polymer with thepowder-like agglomeration auxiliary agents by heating of the mixture.The heating is carried out in agitating or in rest position of thelayer. Several methods are available for the heating or the powdermixture, e.g., by contact with the heated surfaces or hot gases, byirradiation heat, or high-frequency heating. Examples for suitable andheatable apparatuses are: tumbling mixer and force mixer, heating ovensand rotary kilns, ribbons and vibration channels, and the fluidized-bed.The necessary agglomeration temperature depends on the agglomerationauxiliary agent and theoretically ranges between temperatures of muchbelow 0° C. up to the decomposition temperature of the water-swellablepolymers. It is decisive that the agglomeration auxiliary agent and thewater-swellable polymer are admixed with one another at temperatures atwhich both are present in the form of powders and that subsequently theagglomeration is carried out by heating of the powder mixture. The timeof agglomeration depends on the time needed to heat the powder mixture,i.e., on the heat transition and the temperature difference. Timesbetween 1 minute and 1 hour are necessary--depending on the process,temperature and layer thickness.

The course of the agglomeration may easily be observed in a heatable,transparent tumbling mixer.

The auxiliaries used for the agglomeration are powdery in the range of-100° to +300° C., preferably in the range of 20° to 200° C., and inparticular in the range of 50° to 160° C., and will soften or melt abovethese temperatures. Thus those auxiliaries are suitable as well, whichwill be powdery at temperatures below said temperatures, e.g., -100° C.,and, at room temperature, will e.g., be sticky. Such substances arepulverized by cold milling at low temperatures, at which they arebrittle, are mixed in cold condition with the polymers and subsequentlyheated, e.g., to room temperature.

The amount of powdery agglomeration auxiliary agent depends on its type,grain size and the purpose of use of the granular material. The granularmaterial may contain 75 to 99.5%-wt. of water-swellable polymer, and25.5 to 0.5%-wt of powdery agglomeration auxiliary, respectively.

The auxiliaries used for the agglomeration may additionally containnon-agglomerating compounds such as, fillers, softeners, flow improvers,separating agents, hardeners, antistatic agents, stabilizers, and/orfoaming agents.

After agglomeration, the granular material is sieved and the desiredgrain fraction, which may be compared to an ungranulated water-swellablepolymer having the same grain size, is tested with respect to itsabsorption rate in a solution of sodium chloride. The oversize is groundand the undersize is returned.

The present invention further relates to disposable products forhygienic articles, like diapers and sanitary napkins comprising asabsorbent for body liquids, such as water and urine, a granular materialaccording to claim 1.

Test:

The amount of liquid absorbed is measured per gram of water-swellablepolymer (granulated polymer).

The weighed amount of polymer is enclosed in a tea bag and dipped in a0.9% sodium chloride solution. The amount of absorption is determinedafter

a) 15 seconds and

b) 5 minutes with subsequent centrifugation at 1400 rpm,

whereby the liquid amount absorbed by the material of the tea bag issubtracted. ##EQU1## It is found out that the agglomerates areconsiderably superior to both an non-agglomerated, water-swellable,powdery polymer of comparable grain size and the fine grain used asstarting material of the agglomeration, especially with respect to therate at which the liquid is absorbed.

Comparative polymers

Comparative polymer (commercial)

Type: cross-linked solvent polymer based on acrylic acid which ispresent at 70% per mole as sodium salt

grain fraction: 150-630 μm

Absorption after 15 sec: 12 g/g

Absorption after 5 min: 39 g/g

EXAMPLE 1

95 g of the comparative polymer having a grain size of smaller than 90μm are mixed in a high-efficiency mixer for 5 minutes with 5 gcopolyester having a grain size of 60-200 μm, a melting range of115°-125° C., a melting index of 34 g/10 min. ¹) and a melting viscosityof 3600 dPas at 160° C. In order to agglomerate the powder mixture istransferred into a heatable glass tumbling mixer. After 2 minutes, at awall temperature of 160° C., the start of the agglomeration can easilybe seen. 5 minutes later, the granular material is taken-off and dividedinto grain fractions of smaller than 150 μm, 150-630 μm, and larger than630 μm within the air-flow classifier. The main friction of 150-630 μmis tested:

Yield of granular material: 85%

Absorption after 15 sec.: 37 g/g

Absorption after 5 min.: 41.8 g/g

EXAMPLE 2-6

Process as in example 1. The comparative polymer having a grain sizeof<90 μm is used for the agglomeration.

    __________________________________________________________________________    Agglomeration                          test                                   auxiliary                              granular mat. 150-630 μm                              grain-                                                                            melting-                                                                           melting-                                                                           conditions                                                                           absorption                                                                           absorption                           amount       size                                                                              range                                                                              index                                                                              time                                                                             temp.                                                                             after 15 sec.                                                                        after 5 min.                    example                                                                            %   Name     μm                                                                             °C.                                                                         g/10 min.                                                                          min.                                                                             °C.                                                                        g/g    g/g                             __________________________________________________________________________    2    5   Copolyamide.sup.4)                                                                     0-80                                                                              115-125                                                                             15.sup.1)                                                                         8  160 28.8   41.3                            3    5   HD-polyethylene                                                                         1-300                                                                            100-105                                                                            200.sup.2)                                                                         5  160 35.9   36.4                            4    5   Ethylene-Vinyl-                                                                         1-300                                                                            70-80                                                                              160.sup.1)                                                                         10 160 23.2   37.2                                     acetate copolymer                                                    5    5   HD-Polyethylene                                                                        5-74                                                                              109-112                                                                             7.sup.3)                                                                          10 160 45.0   41.5                            6    5   Co-Polyamide.sup.5)                                                                    0-80                                                                              80-90                                                                               65.sup.1)                                                                         8  160 36.1   42.1                            __________________________________________________________________________     HD is high density                                                            MFI is melt flow index                                                        .sup.1) MFI 160° C. / 2.16                                             .sup.2) MFI 190° C. / 2.16                                             .sup.3) MFI 190° C. / 2                                                .sup.4)   ®Platamid H 005, Deutsche Atochem, Bonn                         .sup.5)  ®Platamid H 103, Deutsche Atochem, Bonn                     

EXAMPLES 7-15

The comparative polymer having the grain size of smaller than 90 μm isadmixed with the powdery agglomeration auxiliary agent, and the powdermixture is stored in the drying oven at a layer-thickness of 4 cm for aperiod of 30 min. After cooling, the powder cake is crushed and sieved.

    __________________________________________________________________________    Agglomeration                                                                                                   Granular mate-                              Auxiliary                         rial 150-630 μm                          agent                             Absorption                                       Amount                   Temp.                                                                             after 15 sec.                               Example                                                                            %    Name                °C.                                                                        g/g                                         __________________________________________________________________________     7   5    90 ethylene oxide units-stearic acid                                                              160 36.1                                         8   5    polyethylene oxide  225 37.8                                                  mol-wt. (number average) about 4 × 10.sup.6                    9   5    Calcium laurate     160 27.6                                        10   5    high-press. polyethyl.                                                                            160 35.9                                        11   2    co-polyamide.sup.1) 160 28.6                                                  melting range                                                                 110-120° C.                                                            grain size up to 80 μm                                           12   5    "                   130 33.5                                        13   5    co-polyamide.sup.2) 160 42.5                                                  melting range                                                                 115-130° C.                                                            grain size 0-80 μm                                               14   5    terpolymer from     130 29.5                                                  acrylic acid, ethylene                                                        acrylic acid ester                                                            grain size 5-74 μm                                                         MFI 190/2: 7 g/10 min                                                         density: 0.929 g/cm.sup.3                                                     melt. range.: 98-102° C.                                     15   5    ethylene vinylacetate-                                                                            160 31.5                                                  copolymer                                                                     grain size 5-74 μm                                                         MFI 190/2: 4 g/10 min                                                         melt. range: 92- 95° C.                                      __________________________________________________________________________     .sup.1)  ®RGriltex 1, Emser Werke AG                                      .sup.2)  ®RPlatamid M 840 PA, Deutsche Atochem, Bonn, FRG,           

EXAMPLE 16

A heatable screw mixer is loaded with 14.25 kg comparative polymer ofthe grain size of smaller than 90 μm and 0.75 kg pulverized citric-acidmonohydrate. After mixing at room temperature for 10 min., the walltemperature is increased to 160° C. by heating with vapour. After 8min., the granular material is taken off, cooled and sieved.

The grain fraction 150-630 μm has an absorption rate for 0.9% sodiumchloride solution of 26 g/g in 15 seconds.

EXAMPLE 17

A powder mixture of 180 g comparative polymer having a grain size ofsmaller than 150 μm and 20 g copolyamide powder (as in Example 6) isstored for 3 minutes in a commercial microwave oven having an connectionvalue of 1400 watts. The final temperature is 95° C. After cooling, thepowder cake is crushed and sieved.

The grain fraction 150-630 μm has an absorption rate for 0.9% sodiumchloride solution of 28.5 g/g of granular material in 15 seconds.

EXAMPLE 18

1.54 kg of the powder mixture as in Example 15 are put in a fluid bed.The layer height in rest position is 100 mm. At an air velocity of 0.33m/s and an inlet air temperature of 180° C., after 28 min. The granularmaterial yield of the grain fraction 150-630 μm is 86.5%-wt. Thisgranular material has an absorption rate for 0.9% sodium chloridesolution of 33.5 g/g of polymer in 15 seconds.

EXAMPLES 19-21

Comparative polymer of the grain size of smaller than 150 μm is admixedwith the powder-like agglomeration auxiliary agent and treated in thefluidized-bed according to example 18.

    __________________________________________________________________________                                     Test                                                                          Granular mate-                               Agglomeration                    rial 150-630 μm                           Auxiliary                        Absorption                                        Amount                  Temp.                                                                             after 15 sec.                                Example                                                                            (%)  Name               (°C.)                                                                      (g/g)                                        __________________________________________________________________________    19   5    polyurethane       140 29.3                                                   melting range: 160-180° C.                                             grain size: 0-500 μm                                                       Shore-A-hardness:                                                             82 ± 2 acc. to DIN 53505                                         20   5    polystyrene        160 25.0                                                   MFI 200/5:25 g/10 min.                                                        grain size: 5-90 μm                                              21   5    low pressure, low density polyethylene                                                           160 30.2                                                   MFI 190/2:10 g/10 min.                                                        grain size: 5-90 μm                                                        melting range: 131-134° C.                                   __________________________________________________________________________

EXAMPLE 22

A powder mixture of 1455 g comparative polymer having the grain size ofsmaller than 100 μm, 45 g high-pressure polyethylene of the grain size5-74 μm, the density of 0.924 g/cm³ and the melting index of 7 g/10 min(MI 190/2) and 1.5 g Aerosil 200³) are treated in the fluid bed untilthe product temperature of 140° C. is achieved. The granular materialyield is:

    ______________________________________                                        16.3% of the grain size                                                                           larger than 630 μm                                     82.2% of the grain size                                                                           150-630 μm                                              1.5% of the grain size                                                                           smaller than 150 μm                                    ______________________________________                                    

The grain fraction 150-630 μm has an absorption rate of 25.4 g 0.9%sodium chloride solution per gram of granular material in 15 seconds.

The following test is to demonstrate the surprising concentration of thesintered granular material in swollen condition. This concentration ofthe swollen gel is of great importance for the use of water-swellablepolymers, e.g., for diapers. The higher the concentration of the gel,the lower the danger of exudation from the diapers.

The grain fraction of 200-300 μm is tested according to the Demandabsorbency test, described in: "Allgemeiner Vliesstoff-Report", May1987, pages 210-218, and the swollen gel is tested as to itsconcentration after reaching the maximum absorption.

5 g of max. swollen gel is put in a 10 ml pipette equipped with a pistonand a 300-μm sieve which is fixed at its bottom. The pressure at whichthe gel is pressed through the openings of the sieve is determined underincreasing weight load of the piston. The load on the piston of thepipette is expressed in g per cm².

    ______________________________________                                                         Max. Amount of                                               Product acc.     Liquid absorbed                                                                            Load                                            Example No.      ml/g         g/cm.sup.2                                      ______________________________________                                        comparative polymer                                                                            67           700                                              5               56           900                                             14               55.1         1060                                            15               57           760                                             ______________________________________                                    

I claim:
 1. A process for the manufacture of granular material whichcomprisesa) mixing water-swellable polymer particles of from 10 to 150μm with an agglomeration auxiliary agent which is powdery in the rangeof -100° to +300° C., the water-swellable polymer particles beingpresent in from 75 to 99.5% by weight and the agglomeration auxiliaryagent particles being present in from 25 to 0.5% by weight, b) heatingthe mixture for a time between one minute and one hour sufficient torender the auxiliary agent particles adhesive, thereby agglomerating atleast about 85% by weight of the water-swellable polymer particles intoparticles of about 150 to 630 μm, c) cooling, and d) optionallyclassifying the granular material substantially to remove therefromparticles outside the range of about 150 to 630 μm.
 2. A processaccording to claim 1, wherein the agglomeration auxiliary agent has amelting/softening point temperature range.
 3. A process according toclaim 1, wherein the auxiliary agglomerating agent comprises at leastone of a polyolefin, polyamide, polyester, poly(meth)-acrylate,poly(meth)-acrylonitrile, polyalkylene oxide, polyvinylchloride,polystyrene, polycarbonate or polyurethane.
 4. A process according toclaim 1, wherein the auxiliary agent comprises at least one of anepoxide resin, aminoplast or phenoplast, optionally in combination witha hardener.
 5. A process according to claim 1, wherein the auxiliaryagent comprises at least one of a cellulose ester, shellac, colophony orderivative thereof, or a natural thermoplast based on a fatty acid orderivative thereof.
 6. A process according to claim 1, wherein there isadditionally included in the materials being heated at least one of anon-agglomerating filler, softener, flow improving agent, separatingagent, hardener, antistatic agent, stabilizer or foaming agent.
 7. Aprocess according to claim 1, wherein the water-swellable polymercomprises at least one of a polymer of (meth)-acrylic acid, a polymer ofat least partially saponified (meth)-acrylonitrile, a polymer of atleast partially saponified (meth)-acrylamide, a polymer of an at leastpartially saponified (meth)-acrylic acid ester, a graft copolymer of(meth)-acrylonitrile or (meth)-acrylamide with starch or cellulose or aderivative thereof, a polyurethane and a polymer of isobutylene andmaleic acid anhydride.
 8. A process according to claim 1, wherein thewater-swellable polymer comprises an ionogenic or non-ionic monomer. 9.A process according to claim 1, wherein the water-swellable polymer ispresent at least partially as an ammonium, alkali or alkaline earthmetal salt.
 10. Granular particles produced by the process of claim 1.11. Granular material of water-swellable polymers according to claim 10at least 85% of which is from 150 to 630 μm in size, characterized by acontent of water-swellable polymer of 75 to 99.5%-wt, and 25 to 0.5%-wtof an agglomeration auxiliary agent as main component, whichagglomeration auxiliary agent is a powder and is capable ofagglomerating the finely divided water-swellable polymer particles toparticle sizes which can be used as absorbent for practical purposes,and optionally further additional non-agglomerating components.